Your boiler is operating normally and there are no faults detected.

Appliance is ok

What causes this fault?

This is actually the normal operating state of your boiler and does not indicate a breakdown or failure. The light is triggered by the boiler receiving a signal to provide heating or hot water, or it simply indicates that the system is in standby mode and ready to work. There is no physical fault present; the internal components are simply communicating that they are powered and functioning as intended.